In general, radio frequencies are roughly divided into two categories, namely licensed frequency bands and unlicensed frequency bands. Licensed frequency bands represent frequencies which are often tightly controlled, while unlicensed frequency bands are available for use by many entities with little or no regulation. Furthermore, unlicensed frequency bands include the so-called “industrial, scientific, and medical” (ISM) bands.
However, a concerned issue is that the unlicensed frequency bands have become quite crowded. This is due to many factors, including the rapid increase of wireless devices in the ISM bands, the explosively enlarging popularity of 802.11, or the expansion of Wi-Fi. An approach is to allow unlicensed devices to access licensed frequencies or frequency bands as long as licensed users are not engaging the licensed frequencies or frequency bands.
The spectrums or portions which are not in active use by licensed users are called white spaces and cognitive radio is one emerging technique to facilitate the use of white spaces. Moreover, cognitive radio serves to change the transmission and reception parameters to avoid interference with other devices and it often requires an efficient medium access control layer to utilize the potentially dynamic white spaces. Another concerned issue is that the conventional medium access control layer is performed in contention access mechanism and it leads to low spectrum efficiency.
There is a need to solve the above deficiencies/issues.